The present invention is directed to capacitive touch sensors and, more particularly, to a capacitive keypad position sensor with multiplexed electrodes that reduces or eliminates interference among inaccurate touch inputs.
Touch panels have become prevalent in recent years, particularly in the field of mobile devices, such as cell phones, tablets, and the like. Touch panels may utilize capacitive detection to determine an input. That is, the touch panel includes an insulative material, such as glass, that is coated on one surface with one or more electrodes. A user's finger pressed upon the uncoated surface of the insulator forms a capacitor with an adjacent electrode, causing a disturbance in the electric field applied to the electrode that is detected and registered as the position on the panel touched by the user.
Often, such touch panels will include one or more defined key areas, which can represent alphanumeric or other data input. If there is a one-to-one correspondence between electrodes and defined key areas, as the number of defined key areas increases, the number of electrodes, and hence the number of inputs to a microcontroller or like processor also increases. This can increase complexity and cost for manufacturing the device.
One solution that has been developed is touch key multiplexing, which allows the number of electrodes to be less than the number of defined key areas on the touch panel. In the multiplexing configuration, each defined key area is assigned a unique combination of two electrodes such that the touch by a user of a defined key area alters the electric field of two electrodes simultaneously. The microcontroller determines the proper input based on the combination. Thus, electrodes can be used for multiple defined key areas and the number of electrodes required may be reduced.
FIG. 1 shows an array 100 of defined key areas 101-112 that represent a sequence of twelve numbers key1-key12 selectable by a user. The array 100 further includes ten electrodes E1-E10. A unique combination of two of the electrodes is assigned to each of the defined key areas 101-112, with each electrode E1-E10 occupying about half of an area below the respective defined key area 101-112.
Unfortunately, this design has a drawback in that the user must touch near the center of the desired defined key area 101-112 in order to actuate the proper combination of electrodes E1-E10. If the user's finger lands too far to one side of the center of the desired defined key area 101-112, the microcontroller may not properly register the input. Worse, as shown in FIG. 1 by the errant touches 82, 84, a different defined key area 101-112 may be registered by the microcontroller than the desired input. This phenomenon is called cross-interference between keys.
For example, if the user's finger 82 lands between key4 104 and key7 107, the electrodes E5 and E6 would be actuated, and the microcontroller, based on the received input combination of electrodes E5 and E6 will register that key12 112 (having the combination of electrodes E5 and E6) was selected. Similarly, if the user's finger 84 lands between key2 102 and key3 103, the electrodes E1 and E4 are actuated, which the microcontroller will understand as selection of key8 108.
It is therefore desirable to provide a touch panel that reduces or eliminates the potential for cross-interference between adjacent keys when the user's touch is not precisely centered on the desired defined key area.